348 Willis .— The Sources and Distribution of the 
speak in favour of age and area. Further, as I have pointed out above, 
prediction is possible under age and area, and impossible under Natural 
Selection, and, so far. all predictions made have been verified by the actual 
facts. 
The objection to c age and area ’ that many people profess, when 
analysed, is really an objection to changing the mode of looking at certain 
facts. But, as I have already pointed out, the new method is a priori just 
as likely to be correct as the old, and in any case it is a good thing some¬ 
times to change one’s point of view, even if it be only for a time. 
Other Factors than Age active. 
Dr. Sinnott objects that I have not allowed enough for the action 
of other factors than age. I have never pretended to exclude them from 
operation ; in my paper on Ceylon ( 18 ), p. 5, I wrote, ‘ of course in the case 
of any single species numerous disturbing influences come into play ’, and 
in the New Zealand paper I called attention to the effects of man’s action, 
change of climate, &c. In my reply to Mr. Ridley (20), which Dr. Sinnott 
had not seen when he wrote, I have gone more fully into the question of 
other determinative factors in distribution, and have given a list of such 
factors, which is being steadily added to. 
The figures I have given, however, show that age and area is a law which 
appears to affect all plants taken in groups of allied species more or less 
closely alike, and this cannot be said of any of the other causes. Probably 
in every single case of an individual species one or more of them come into 
action ; they do not, however, act alike on groups of species, but pull every 
way, so that when groups of allied species are considered, as should always 
be the case in using c age and area ’, their results cannot be clearly seen. 
Hydrogen rises in air; an aeroplane rises; a piece of paper falls 
slowly ; a rifle bullet falls at an angle with the soil; yet the law of gravity 
is recognized as universally applicable in all these cases, though other 
factors may be acting so strongly as to conceal its operation. No one 
doubts the validity of Mendel’s law because in almost no single case does 
the progeny appear in the exact proportions required by the law ; and 
similarly with age and area, I believe it to be a general law, though 
its action may be concealed fn individual cases by the operations of one 
or more of the other factors which have some effect in determining 
geographical distribution. 
Age and area shows just as clearly in the Ferns as in the Angiosperms, 
and in both shows family by family and genus by genus, so that it is 
obviously a very ancient law, whereas the other factors are mostly such as 
only come into operation in individual cases, or, as in the case of Natural 
Selection, exert a differentiating action, with results which do not show 
in the figures for geographical distribution. 
